Solid ink or phase change ink printers conventionally receive ink in a solid form, either as pellets or as ink sticks. The solid ink pellets or ink sticks are typically inserted through an insertion opening of an ink loader for the printer, and the ink sticks are pushed or slid along the feed channel by a feed mechanism and/or gravity toward a heater plate in the heater assembly. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a melt reservoir. The melt reservoir is configured to maintain a quantity of melted ink in liquid or melted form and to communicate the melted ink to one or more printheads as needed.
In order to prevent the ink storage and supply assembly of the imaging device from exhausting the available supply of ink, the reservoirs of the ink storage and supply assembly may be provided with ink level sensors. Recently, ink level sensors have been developed that enable a continuous measurement of the level of ink in the reservoirs of the printer. These ink level sensors include a lower probe positioned near a lower portion of the reservoir, an upper probe that extends upward form the lower probe toward the top of the reservoir, and an outer probe. To detect the level of ink in an ink reservoir, an AC signal is driven to the outer probe. The ink in the reservoir conducts the AC signal to the lower probe and to the upper probe. A current flow is detected from the outer probe through the ink to the lower probe and from the outer probe through the ink to the upper probe. Assuming that the ink temperature and conductivity remains relatively consistent, a substantially constant current flow is detected via the lower probe. Varying levels of current flow are detected via the upper probe as more or less of the upper probe's surface area is covered or uncovered in ink. A continuous measurement of the height of ink in the ink reservoir may then be determined by calculating the ratio of the varying current flow in the upper probe to the constant current flow in the lower probe.
One difficulty faced in the operation of the level sensors described above is ink from the reservoir contacting the root of one or more of the probe tabs or the probe connection points and shorting out the signal path from the outer probe to one or both of the upper and lower probes, effectively bypassing the probes in the reservoir. Ink shorting out the probe signal paths can cause the level sensing system to fail in multiple ways including, for example, false high readings, false low readings, and shorts, any of which may ultimately disable the printer and require a field replacement of the melt reservoir system.